1. Field of the Invention
The present invention relates to a particle counting method whereby scattered light produced by radiating a fluid sample with a laser light is detected to count the number of particles for each particle size range.
2. Description of the Prior Art
As a conventional method for counting particles in a liquid or in air, a method is known whereby the magnitude of the output voltage of a photoelectric conversion element receiving scattered light caused by particles is analyzed to find the size of particles in a liquid sample, thereby counting the number of particles for each particle size range.
Further, a method is known whereby the number concentration is determined by the time when the output voltage of a photoelectric conversion element receiving scattered light exceeds a predetermined threshold (for example, refer to Patent Document 1). Another method is known whereby the duration time of scattered light is measured to discriminate the scattered light by the particles from a background light (for example, refer to Patent Document 2). A still further method is known whereby a sampled latest value is sequentially compared to a preceding value and, when it is judged that the latest value is smaller than the preceding value, the scope of the particle sizes is searched referring to a particle size table, thereby counting the particles for each particle size range. However, when the pulse width exceeds a predetermined time, the particles are not counted (for example, refer to Patent Document 3).
Patent Document 1: Japanese Patent Application Publication No. H09-273987
Patent Document 2: Japanese Patent Application Publication No. S63-149541
Patent Document 3: Japanese Patent Application Publication No. H11-258145
However, particles moving near an inner wall of a flow cell or particles moving near a flow channel boundary of a flow channel formed as a result of the flow of an air sample are slower than those moving through the center of the flow channel and exhibit unstable behavior. In this manner, there is a problem in that, if a judgment is made only by the time when the output voltage of a photoelectric conversion element exceeds a threshold or by the duration time of scattered light, one particle may be falsely recognized as a plurality of particles or particles may not be counted.
Further, a signal wave form by a large particle is accompanied by an overshoot. In this manner, if a judgment is made only by the time when the output voltage of the photoelectric conversion element exceeds the threshold or by the duration time of the scattered light, there is a problem in that one particle may be falsely recognized as a plurality of particles.